In this article, an approach based on an array of macro-fiber composite (MFC) transducers arranged as rosettes is proposed for high-velocity impact location on isotropic and composite aircraft panels. Each rosette, using the directivity behavior of three MFC sensors, provides the direction of an incoming wave generated by the impact source as a principal strain angle. A minimum of two rosettes is sufficient to determine the impact location by intersecting the wave directions. The piezoelectric rosette approach is easier to implement than the well-known time-of-flight-based triangulation of acoustic emissions because it does not require knowledge of the wave speed in the material. Hence, the technique does not have the drawbacks of time-of-flight triangulation associated to anisotropic materials or tapered sections. The experiments reported herein show the applicability of the technique to high-velocity impacts created with a gas-gun firing spherical ice projectiles.
Heart rate (HR) as an important physiological indicator could properly describe global subject’s physical status. Photoplethysmographic (PPG) sensors are catching on in field of wearable sensors, combining the advantages in costs, weight and size. Nevertheless, accuracy in HR readings is unreliable specifically during physical activity. Among several identified sources that affect PPG recording, contact pressure (CP) between the PPG sensor and skin greatly influences the signals. Methods: In this study, the accuracy of HR measurements of a PPG sensor at different CP was investigated when compared with a commercial ECG-based chest strap used as a test control, with the aim of determining the optimal CP to produce a reliable signal during physical activity. Seventeen subjects were enrolled for the study to perform a physical activity at three different rates repeated at three different contact pressures of the PPG-based wristband. Results: The results show that the CP of 54 mmHg provides the most accurate outcome with a Pearson correlation coefficient ranging from 0.81 to 0.95 and a mean average percentage error ranging from 3.8% to 2.4%, based on the physical activity rate. Conclusion: Authors found that changes in the CP have greater effects on PPG-HR signal quality than those deriving from the intensity of the physical activity and specifically, the individual best CP for each subject provided reliable HR measurements even for a high intensity of physical exercise with a Bland–Altman plot within ±11 bpm. Although future studies on a larger cohort of subjects are still needed, this study could contribute a profitable indication to enhance accuracy of PPG-based wearable devices.
In this paper (Part I) the use of fiber optic sensors for real-time monitoring of the cure
kinetics of GFRP composites is explored. The proposed sensing system allows the
simultaneous measurement of both temperature and strain by monitoring the change in
reflected wavelength from two coupled fiber Bragg grating (FBG) sensors that
have been embedded into the composite laminate. Instrumented GFRP laminates
with 12, 18 and 24 reinforcing plies, respectively, were prepared by means of the
vacuum bagging technique. Samples were cured in a thermally controlled oven at
80 °C
and 30 kPa for 240 min (isothermal stage) and then cooled down to ambient
temperature by turning off the heating source (cooling stage). The obtained results,
combined with proper data post-processing, have proven the effectiveness and
potentiality of the proposed sensing system to measure the progression of the
composite cure kinetics. It was shown that temperature within the specimen
can differ significantly from the set-point temperature inside the oven because
of the heat released during the exothermal reticulation of the epoxy resin. The
combined sensing system also allowed the residual strain accumulated within the
composite during the cooling stage to be accurately measured. Once the laminate had
been cured, the embedded optical sensing system reveals itself purposeful for
real-time structural health monitoring and damage assessment of the finished
component. This aspect is discussed with more detail in the accompanying paper (Part
II).
In this paper we present an optical technique based on the
shadow moiré method which allows the measurement and
digitization of three-dimensional surfaces. The technique was
tested through experimental work and the results were compared with
those obtained by a coordinate measuring machine. Moving from the
conventional shadow moiré method, new features were implemented
enabling us to overcome the main shortcomings of the conventional
moiré method. These include the need to assign the fringe order,
the incapability of discerning concavity or convexity, the poor
resolution and the complexity in the signal processing. All these
problems have been solved by adding an element to generate a carrier
fringe pattern to the equipment of the conventional shadow moiré
technique and processing the obtained signal using the Fourier
transform method. The proposed technique was applied to obtain
external surfaces of sheet metal stamped parts. The experimental
results show the effectiveness of this technique.
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